(1) Field of the Invention
This invention relates to an image scanning method for compensating for a pyramidal error of a polygon mirror by fine-adjusting an angle of a light beam incident on the polygon mirror and to an image scanning apparatus for carrying out such a method.
(2) Description of the Prior Art
In a laser beam scanning method using a polygon mirror, the laser beam is generally incident on a mirror surface of the polygon mirror from a direction which makes a certain angle with an optical axis of a scanning lens such as an f.multidot..theta. lens and belongs to a main scanning plane. The main scanning plane is orthogonal to a rotating axis of the polygon mirror and includes the optical axis in this specification.
When the polygon mirror has a pyramidal error, the laser beam reflected on the mirror surface is deviated and led to outside the main scanning plane. A locus of the beam on a plane to be scanned (namely, a line made of dots formed on the plane in the main scanning direction; will be referred to as a trace line) is deviated in a sub scanning direction or curved. Therefore, the trace lines have poor parallelism and non-uniform pitches, which deteriorates image quality drastically.
The above problem is solved by compensating for the pyramidal error.
An angle made of the light beam reflected from the polygon mirror and the main scanning plane is varied by a rotating angle of the rotating axis of the polygon mirror even on the same trace line. Accordingly, a highly precise compensation for the pyramidal error requires a total pyramidal error to be considered. The total pyramidal error means the combination of a "static" pyramidal error which is inherent in manufacturing the polygon mirror and an "active" pyramidal error which is influenced by the rotating angle of the rotating axis.
If the "active" pyramidal error is not considered, the rotation of the polygon mirror accompanies constant changes in the angle made of the reflected beam and the main scanning plane, whereby a focusing point of the reflected beam is changed. Accordingly, the trace line is asymmetrically curved with respect to the ideal scanning line which corresponds with the X axis of FIG. 1.
Japanese Patent Publication Kokai No. 58-100117 has disclosed a technology for compensating for the total pyramidal error. This publication teaches obtaining the relationship between a pyramidal error .delta. and a deviation amount .DELTA. of the trace line from the ideal scanning line on the plane to be scanned in the sub scanning direction.
This is not enough to compensate for the total pyramidal error for the following reason.
In order to eliminate the deviation or the curve of the trace line, the angle of incidence of the light beam on a polygon mirror should be adjusted so that the deviation amount .DELTA.=0, namely, the pyramidal error .delta. is eliminated, whereby keeping the reflected beam within the main scanning plane. Such an adjustment is carried out by 1) obtaining the relationship among the pyramidal error .delta., an angle made of a normal line on the mirror surface and the optical axis of the scanning lens, the angle made of the incident beam and the optical axis, and the angle made of the incident beam and the main scanning plane and then 2) controlling the angle of incidence on the polygon mirror so as to realize .DELTA.=0 based on the above-obtained relationship.
However, this publication does not teach any practical equation indicating the above relationship. Therefore, it is practically impossible to carry out a constant real-time compensation in synchronization with the rotation of the polygon mirror according to this publication.